E.g. http://forum.frozenplague.net/users/anonymous
=> Standard "We're sorry, but something went wrong." message.

For example:
app/views/layouts/application.html.erb should have

<link rel='stylesheet' href='<%=ActionController::Base.relative_url_root%>/themes/<%= theme.name %>/style.css'>

instead of:

<link rel='stylesheet' href='/themes/<%= theme.name %>/style.css'>

app/helpers/application_helper.rb#theme_image_tag also needs this.

style.css should use relative links to images:
e.g. public/themes/blue/style.css

.title-bar { background: transparent url(themes/blue/bg.jpg) repeat scroll 0%; ... }

should be
.title-bar { background: transparent url(bg.jpg) repeat scroll 0%; ... }

These changes allow you to run the server at a context other than /

Line 3 of environment.rb forces the development environment. Is there any reason for this? Since it is development caching is not turned on and that may be a performance concern.